If your domain name just redirects to wordpress.com, then there is nothing you can do. When the user is viewing one of your posts, they are viewing it on wordpress.com, so that is the link that is shared. This is a compromise you accepted when you set up your blog the way you did.
If you want to change it, you can either:
- Host the blog yourself instead of using wordpress.com.
- Use WP’s premium Domain Mapping feature. This would be the easiest for you, but it costs $12.00 per year.